Method and apparatus for loosely sychronizing closed free-running raster displays

ABSTRACT

The present invention is a means and method for synchronizing closed free-running systems, such as graphics systems, with no external synchronization signals required. Video games and most computer display controllers are closed free-running systems. Because most such systems have the means to switch between an interlaced and non-interlaced operation, and because interlaced and non-interlaced modes have a relative timing variation, the timing between two or more such closed free-running systems can be synchronized. This method allows synchronization with an imprecise timing reference. The vertical display timing is the free-running oscillator and the interlaced/non-interlaced mode transition is used as the timing adjustment means. The actual arrival time of data in a communication medium connecting two systems being synchronized is used in relation to an expected arrival time to provide the clock reference.

This is a continuation of application Ser. No. 08/334,676, filed Nov. 4,1994, now abandoned.

FIELD OF THE INVENTION

The present invention relates to the field of raster scan displaycontrollers. Specifically, the present invention pertains tosynchronization of multiple raster scan display controllers.

REFERENCE TO RELATED PATENT APPLICATIONS

The following co-pending patent applications are related:

U.S. patent application entitled, "AN IMPROVED NETWORK ARCHITECTURE TOSUPPORT REAL-TIME VIDEO GAMES", invented by Steve Perlman, with Ser. No.08/238,630 and filed on May 5, 1994.

U.S. patent application entitled, "AN IMPROVED NETWORK ARCHITECTURE TOSUPPORT MULTIPLE SITE REAL-TIME VIDEO GAMES", invented by Steve Perlman,with Ser. No. 08/238,477 and filed on May 5, 1994.

U.S. patent application entitled, "AN IMPROVED MODEM TO SUPPORT MULTIPLESITE CALL CONFERENCED DATA COMMUNICATIONS", invented by Steve Perlman,with Ser. No. 08/238,456 and filed on May 5, 1994.

U.S. patent application entitled, "AN IMPROVED NETWORK ARCHITECTURE TOSUPPORT RECORDING AND PLAYBACK OF REAL-TIME VIDEO GAMES", invented bySteve Perlman, with Ser. No. 08/238,303 and filed on May 5, 1994.

DESCRIPTION OF RELATED ART

Images are drawn on prior art raster display systems, such as televisionand computer displays, by tracing a plurality of horizontal raster scanlines, each scan line comprising a row of individual pixels. The entireimage is scanned out sequentially by a video controller one scan line ata time from the top left corner of the display screen to the bottomright corner of the display screen. Clocking circuitry, typicallyincluded with the video controller, is used to maintain precise controlover the rate at which scan lines are traced. Typically, a crystaloscillator is used as a clock source for this clocking circuitry.Although oscillators and other clock sources usually provide a highlyaccurate clock source for a particular display system, small variationsin the timing between different oscillators invariably occur. Thesevariations can be aggravated by environmental conditions such astemperature. Thus, it can be expected that two identical, butindependent, display systems initially started at the same instant willeventually drift out of synchronization to the point where one of thedisplay systems will eventually get a full frame ahead of the other.These prior art systems do not provide a means for synchronizing theraster scan process among a plurality of independent raster displaysystems without driving each system with a precise common clock source.

Prior art raster display systems operate in two fundamental types ofrefresh modes: interlaced and non-interlaced mode. Interlaced mode isused in broadcast television (NTSC, PAL, and SECAM) and in rasterdisplays designed to drive standard television monitors. NTSC (NationalTelevision System Committee), PAL (Phase Alternate Line), and SECAM arewell known raster display design and operational standards. For NTSC,the refresh cycle in interlaced mode is broken into phases (known as"fields"), each phase lasting 1/60 of a second (1/50 of a second for PALand SECAM); thus, a full NTSC refresh cycle lasts 1/30 of a second (1/25of a second for PAL/SECAM). All odd numbered scan lines are displayed inthe first phase and all even numbered scan lines are displayed in thesecond phase. The purpose of the interlaced scan mode is to place somenew information in all areas of the screen at a 60 Hz rate; because, a30 Hz refresh rate tends to cause an irritating flicker. The net effectof interlacing is to produce a picture whose effective refresh rate isperceptively like 60 Hz while actually running at 30 Hz. This techniqueworks as long as adjacent scan lines display similar information. Animage consisting of dissimilar horizontal lines on alternating scanlines, such as often occurs in computer-generated images, causes anunpleasant line flicker effect.

When an NTSC display is refreshed in a non-interlaced mode (as is commonwith home computers and video games), the refresh cycle consists ofscanning just the first phase or just the odd numbered scan lines at a60 Hz rate. Alternatively, just the even numbered scan lines arerefreshed at a 60 Hz rate. The unrefreshed scan lines in anon-interlaced mode are displayed as black or absent any image features.The non-interlaced mode is commonly used in home computers and videogame displays because line flicker problems cannot be tolerated.

Most video game displays run independently from any external clocksource. Thus, using prior art synchronization techniques, multiple videogame displays can not be synchronized to each other. Even advanced videogames and computer systems that can accept an external clock source forsynchronization require such clock to be extremely precise; accurate tothe order of one part per 10 million. In many situations, such a preciseclock cannot be feasibly provided (e.g. if only a telephone modemconnection exists between the systems).

Thus, a better means and method for synchronizing closed free-runningsystems is needed.

SUMMARY OF THE INVENTION

The present invention is a means and method for synchronizing closedfree-running systems, such as graphics systems, with no externalsynchronization signals required. Video games and most computer displaycontrollers are closed free-running systems. Because most such systemshave the means to switch between an interlaced and non-interlacedoperation, and because interlaced and non-interlaced modes have arelative timing variation, the timing between two or more such closedfree-running systems can be synchronized. This method allowssynchronization with an imprecise timing reference. The presentinvention is specifically applicable to maintaining framesynchronization between two video games or computer systems connectedvia a modem link.

The present invention is similar to a phase-locked loop. The verticaldisplay timing is the free-running oscillator and theinterlaced/non-interlaced mode transition is used as the timingadjustment means. The actual arrival time of data in a communicationmedium connecting two systems being synchronized is used in relation toan expected arrival time to provide the clock reference.

Television studios have distributed synchronized signals that areaccurate on the order of 1 part per 100 million (i.e. 10 us clockprecision). The present invention operates with timing references asslow as 300 Hz with an accuracy no better than 1 part per 525 (about 2msec clock accuracy). Also, the present invention can achievesynchronization by use of a timing reference which is asynchronous toboth raster timings (e.g. 2400 bps modem bit clock). It is a furtheradvantage of the present invention that the implementation of thesynchronization apparatus of the present invention requires a minoramount of processing to maintain synchronization, and no externalhardware needs to be added to the display controller. It is a furtheradvantage of the present invention that the implementation of thesynchronization apparatus of the present invention does not noticeablydisturb the displayed image while maintaining synchronization. The brieftransitions between alternate timing modes cannot typically be perceivedby a human viewer. Television and other raster devices are not disruptedby the brief transitions between alternate timing modes. It is a furtheradvantage of the present invention that the implementation of thesynchronization apparatus of the present invention works with eithernormally interlaced or normally non-interlaced displays. Normallynon-interlaced displays are switched to interlaced mode briefly tomaintain synchronization. Normally interlaced displays are switched tonon-interlaced mode briefly to maintain synchronization. Because bothdisplay generators are crystal-controlled, the display generators willbe fairly stable relative to each other and will only need brief,periodic adjustments. Typically, several hundred normal frames aredisplayed before a single frame of the alternate mode is switched in. Itis a further advantage of the present invention that the implementationof the synchronization apparatus of the present invention is tolerant tothe loss of a timing reference for short intervals. Often in videogames, for example, the vertical blanking interrupt is turned off forbrief periods of time. This occurs, for example, while a new screen isbeing set up during a transition. Although the timing of the two displaycontrollers may drift during this time, their crystal referencesguarantee they won't drift very far. Thus, several seconds can safelyelapse before vertical blanking interrupts are restored. When verticalblanking interrupts are restored, the present invention pulls the twodisplays exactly into synchronization again. Typically, only two orthree alternate frames (as opposed to the usual one frame) are needed torestore synchronization within tolerance. Finally, it is a furtheradvantage of the present invention that the two or more systems beingsynchronized are not allowed to drift more than one full frame apart.Drift by several scan lines is allowed. The present invention maintainssynchronization of two or more systems within the larger of: 1) the timerequired to scan one line or 2) the accuracy of the communicationsmedium's clock. It is a further advantage of the present invention thatbecause the display subsystems of most personal computers (e.g.current-generation IBM™ PC compatibles and Apple™ Macintosh™) and videogame systems (e.g. Sega™ and Super Nintendo Entertainment System™--SNES)are capable of switching seamlessly between interlace and non-interlacedmodes of operation, the present invention has practical application totens of millions of existing systems. These and other objects andadvantages of the present invention will be apparent as presented in thefollowing detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2, 3A, and 3B are simplified illustrations of prior art rasterscan techniques including use of an interlaced mode and a non-interlacedmode.

FIG. 4 is a flowchart illustrating the processing logic of the presentinvention.

FIG. 5 illustrates a typical system having a plurality of raster displaydevices coupled to a data communication medium.

FIG. 6 illustrates the operation of the present invention when thedisplay systems are synchronized.

FIG. 7 illustrates the operation of the present invention when thedisplay systems have drifted out of synchronization and a timingcorrection is required.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a means and method for synchronizing closedfree-running systems, such as graphics systems, with no externalsynchronization signals required. In the following detailed description,numerous specific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone of ordinary skill in the art that these specific details need not beused to practice the present invention. In other instances, well knownstructures, interfaces, and processes have not been shown in detail inorder not to unnecessarily obscure the present invention.

Referring now to FIG. 1, a raster type display screen 110 is illustratedin simplified form. Raster display screen 110 includes a plurality ofscan lines 112. Each scan line comprises a plurality of pixels (notshown). Using a conventional raster scan process, a full screen scanbegins at the upper left corner of screen 110 at point A. Scanningbegins at point A for the first scan line and continues to point B. Theraster is then set to the beginning of the second scan line as indicatedby dashed line 114 illustrated in FIG. 1. The second scan line is thenscanned horizontally from left to right in the same manner. The sameprocess is used to scan each scan line of the display until the lastscan line is scanned. At that point, the raster will be positioned atpoint C and a vertical blanking signal will be generated. The rasterwill then be reset to point A and the full screen scanning process willbe initiated again for the next refresh cycle. The time required toreset the raster to point A from point C is typically known as thevertical blanking interval (VBI). The time required for the raster toscan from point A to point C is a predetermined fixed length time asdetermined by the scan rate of a particular display system. The fullresolution non-interlaced raster scan process illustrated in FIG. 1 anddescribed above is well known to those of ordinary skill in the art.Such a method is commonly used for high resolution computer displays.

Referring now to FIG. 2, a prior art implementation of a half-resolutionnon-interlaced mode is illustrated in simplified form. A display screen210 supporting a full-resolution interlaced and a half resolutionnon-interlaced mode comprises two sets of scan lines (two fields ininterlaced mode). A first set of scan lines 212, illustrated in FIG. 2,is scanned when display 210 is operating in a half-resolutionnon-interlaced mode. A second set of scan lines 214 is not scannedduring operation in a half-resolution non-interlaced mode. Inhalf-resolution non-interlaced mode operation, the raster begins atpoint D and fully scans the first scan line of the first set of scanlines 212. When the first scan line has been completely scanned, theraster is reset to the beginning of the second scan line of the firstset of scan lines 212 as indicated by dashed line 216 illustrated inFIG. 2. Thus, the raster skips over the first scan line of the secondset of scan lines 214 and each of the remaining scan lines of the secondset of scan lines 214. In this manner, the raster refreshes alternatescan lines from point D to point E. Upon completion of scanning to pointE, a vertical blanking signal is generated and the raster returns forthe next refresh cycle to point D. It will be apparent to one ofordinary skill in the art that the scan time required to refresh scanlines in a half-resolution non-interlaced mode from point D to point Eis half the time required to scan all of the scan lines on display 210.This is because only half of the available scan lines are scanned in ahalf-resolution non-interlaced mode. In this manner, an image displayedon display 210 can be updated twice as fast as compared to afull-resolution non-interlaced display, such as the display 110illustrated in FIG. 1 where all of the scan lines are refreshed for eachcycle (assuming both have an equal number of scan lines and the samehorizontal scan rate). The use of a half-resolution non-interlaced modefor refreshing half of the scan lines of a raster display screen is wellknown to those of ordinary skill in the art. A half-resolutionnon-interlaced mode is commonly used by home video game systems toprovide a flicker-free television image.

Referring now to FIGS. 3A and 3B, operation in a full-resolutioninterlaced mode are illustrated in simplified form (note that inactuality interlaced scanning is somewhat more complex, but the timingshown here reflects the issues relevant to the present invention.Detailed diagrams of interlaced scanning can be found in standardtelevision engineering texts or the EIA RS-170A Standards Proposal.) Infull-resolution interlaced mode, a first set of scan lines (first field)312 are refreshed during a first refresh cycle and a second set of scanlines (second field) 314 are refreshed in a second refresh cycle. FIG.3A illustrates the first refresh cycle and FIG. 3B illustrates thesecond refresh cycle. Referring to FIG. 3A, the interlaced mode refreshcycle begins at point F at the beginning of the first scan line in thefirst field 312. When the raster has completely refreshed the first scanline in the first field 312, the raster is moved to the second scan linein the first field 312 as indicated by dashed line 316 illustrated inFIG. 3A. In this manner, each scan line in the first field 312 isrefreshed until the raster reaches point H. Point H, as illustrated inFIG. 3A, is located at the mid-point of the last scan line of the firstfield 312. By scanning only half of the last scan line of the firstfield 312, the raster is able to move to point G at the initial positionon the first scan line of the second field 314. In this manner, theraster is set up to refresh the second field 314 beginning at point G asillustrated in FIG. 3B.

Referring now to FIG. 3B, the second refresh cycle begins for the secondfield 314 at point G. When the first scan line of the second field 314is refreshed, the raster moves to the beginning of the second scan lineof the second field 314 as indicated by dashed line 318 illustrated inFIG. 3B. In this manner, each of the scan lines of the second field 314are refreshed down to the last scan line of the second field 314. Onlyhalf of the last scan line of the second field 314 is refreshed asindicated by point J. When the raster has reached point J, a verticalblanking signal is generated and the raster returns to the initialposition of the first scan line of the first field 312 as indicated bypoint F where the next refresh cycle begins again. Using this process,alternate scan lines of display 310 are refreshed in alternate refreshcycles in an interlaced mode.

Comparing now FIGS. 2 and 3A/B, it can be seen that for a similarhorizontal and vertical scan rate, one frame of FIG. 2 takesapproximately, but not exactly, the same amount of time to scan as onefield of FIG. 3A or 3B. Operation of a raster display device in aninterlaced mode is well known to those of ordinary skill in the art.Interlaced mode is commonly used by television studio video effectsdevices and home video game systems when displaying high-resolutionimages. An example of interlaced scanning is Sega's "Sonic the Hedgehog3" in 2-player mode. Noticeable flicker is apparent.

Although the use of interlaced mode is well known in the art, a subtletiming variation caused by the transition between an interlaced mode andnon-interlaced mode and the application of this timing variation forsynchronizing two raster display devices is not well known to those ofordinary skill in the art. Because the conventional implementation ofinterlaced mode only refreshes half of the last scan line in a field,the overall time required for an entire refresh cycle in an interlacedmode is offset by the amount of time required to refresh half of onescan line of each field. This offset in the refresh cycle time isdenoted herein as Δt.

Referring again to FIGS. 3A and 3B, the time required to scan from pointK to point H in FIG. 3A and point L to point J in FIG. 3B is Δt. Thus,if R represents the refresh cycle time required for refreshing a displayframe in a non-interlaced mode, the refresh cycle time required forrefreshing a single display field (two fields per frame) in interlacedmode is therefore R-Δt. The refresh cycle time required for refreshing asingle display frame (two fields per frame) in interlaced mode istherefore 2 (R-αt). In other words, because Δt is one half of the timerequired to raster scan a single scan line, the timing variation betweentwo non-interlaced mode frames and a single interlaced mode frame isequal to the time required to raster scan a single scan line (i.e., 2·.5scan lines). The actual duration of R and Δt varies for particulardisplay systems depending on the number of scan lines available, thenumber of pixels per scan line, and the scan rate. Note, however that Δtis relatively small in comparison to the refresh cycle time R requiredfor refreshing a non-interlaced frame or interlaced field.

The timing variation caused by the transition between an interlaced modeand non-interlaced mode is different for various specificimplementations of a raster device. Further, conventional systemssupport several variants of the interlaced and non-interlaced modes. Forexample, NTSC specifies 262.5 scan lines per 60 Hz field in aninterlaced mode of operation. Some video games, personal computers, andother devices alter NTSC standard operation and display either 262 scanlines or 263 scan lines per 60 Hz frame (not field) in a non-interlacedmode of operation. "Field" is a term applicable only to interlaced mode.In a half-resolution non-interlaced mode, only half of the scan linesare displayed for each frame, as illustrated in FIG. 2. Thus, gapsbetween the displayed scan lines will be apparent in this mode. However,most video game systems (e.g. Sega Genesis or Super NintendoEntertainment System) have the means to display the same image data inboth fields in interlaced mode. This mode is sometimes referred to asfield-doubled interlaced mode. This mode has the visual effect of justfilling in the gaps between displayed scan lines and adding a smallelement of jitter or flicker to the image. Many prior art displaysystems also have a field-unique interlaced mode used to displaydifferent image data in each of the two fields in interlaced mode. Thismode produces a high resolution image. Thus, there are often threedisplay modes supported in prior art raster display controllers. Theseraster devices are included in video game displays and in most personalcomputer displays. These three display modes are set forth below. NTSCmodes are described, analogous modes for PAL/SECAM are available as wellin prior art systems.

(i) Non-interlaced--262 or 263 lines/60 Hz frame are provided. Gaps aredisplayed between lines. Note that there are 262 or 263 lines (lessvertical blanking lines) scanned from the frame buffer.

(ii) Field-doubled interlaced--262.5 lines/60 Hz field are provided. Thesame field data is displayed in each field. Visually, this mode is thesame as mode (i) above, except the gaps between lines are filled in andthere is a slightly noticeable jitter. Note there are just 262.5 lines(less vertical blanking lines) scanned from the frame buffer.

(iii) Field-unique interlace--262.5 lines/60 Hz field are provided.Unique field data is displayed in each field. There are 525 scan lines(less vertical blanking lines) scanned from the frame buffer.

By way of example and not limitation, these three display modes producesome timing variations relative to each other as will be illustrated byexample below. Over a course of 1/30 second:

mode (i) scans out 262*2=524 or 263*2=526 scan lines.

mode (ii) scans out 262.52*=525 lines.

mode (iii) scans out 262.5*2=525 lines.

Because the line time (i.e. the time required to raster scan a singlescan line) is fixed at 63.556 usec in a typical prior art NTSC displaydevice, the timing difference between mode (i) and mode (ii) or mode (i)and mode (iii) is 63.556 usec (i.e. one line time) per 1/30 second.Another way of looking at it is, by switching between mode (i) and mode(ii) or between mode (i) and mode (iii), the vertical blanking intervalof the display device can be slewed in time relative to that of anotherdisplay device by 30 scan lines/second. This method of slewing thetiming between display devices can be used, as in the present invention,for synchronizing the operation of two or more raster display devices.In the preferred embodiment, the timing adjustment apparatus can slewthe timing between two or more systems by as much as 30 scan lines persecond for NTSC systems and 25 scan lines/sec for PAL systems. Note thatalthough non-interlace mode and interlace mode may vary in the exactnumber of scan lines from implementation-to-implementation, there mustalways be a timing difference between a half-resolution non-interlacemode and full-resolution interlace mode (be it field-double orfield-unique) because of the requirement to have the last scan line behalf of a line long in order to achieve interlacing.

Clearly, if the normal operation mode for a particular display device ismode (i) (as is true for most video games), a timing adjustmentapparatus can maintain synchronization between two devices byoccasionally switching one device to mode (ii) to maintainsynchronization. Because mode (i) and mode (ii) are visually almostidentical, the switch between mode (i) and mode (ii) is virtuallyundetectable to the human eye. Certainly, switching from mode (i) tomode (ii) once per several seconds is virtually undetectable by thehuman eye.

If the normal operation mode for a particular display device is mode(ii), the timing adjustment apparatus can also maintain synchronizationbetween two devices by occasionally switching one device to mode (i) tomaintain synchronization.

If the normal operation mode for a particular display device is mode(iii), the timing adjustment apparatus can also maintain synchronizationbetween two devices by occasionally switching one device to mode (i). tomaintain synchronization. However, part of the image resolution issacrificed by switching to mode (i) from mode (iii). Nonetheless,because the loss of resolution is no more than one field per severalseconds, this loss is also virtually undetectable to the eye. Thepresent invention includes such a timing adjustment apparatus formaintaining synchronization between two raster display devices byswitching among various available display modes.

Note that televisions and display monitors (and even LCD's) are designedto handle both interlaced and non-interlaced modes. These devicesreadily handle the slight variations in timing with no adverse effects.

Note also that non-interlaced mode is "non-standard" NTSC, and as such,system designers may choose either 262 or 263 as the number of displayedscan lines. Televisions work fine with either number of scan lines. Theonly impact this selection has on the present invention is that thetiming adjustment apparatus switches to the "alternate" display mode inone or the other display device depending on the number of scan linesselected and depending on which display device is getting ahead orfalling behind. The following description describes the various cases.

Case 1: In a first and a second display system, if the normal displaymode is non-interlaced with 263 scan lines/frame and the alternatedisplay mode is interlaced with 262.5 scan lines/field (i.e. the normalmode non-interlaced scan time is longer than the alternate modeinterlaced scan time), the first display system is synchronized with thesecond display system as follows:

a. If the first display system falls behind the second display system(i.e. the VBI of the first display system occurs later than that of thesecond display system), the first display system is switched to thealternate display mode (i.e. the shorter scan time mode) and the seconddisplay system is maintained in the normal display mode.

b. If the first display system gets ahead of the second display system(i.e. the VBI of the first display system occurs sooner than that of thesecond display system), the first display system is maintained in thenormal display mode and the second display system is switched to thealternate display mode (i.e. the shorter scan time mode).

Case 2: In a first and a second display system, if the normal displaymode is interlaced with 262.5 scan lines/field and the alternate displaymode is non-interlaced with 263 scan lines/frame (i.e. the normal modeinterlaced scan time is shorter than the alternate mode non-interlacedscan time), the first display system is synchronized with the seconddisplay system as follows:

a. If the first display system falls behind the second display system(i.e. the VBI of the first display system occurs later than that of thesecond display system), the first display system is maintained in thenormal display mode and the second display system is switched to thealternate display mode (i.e. the longer scan time mode).

b. If the first display system gets ahead of the second display system(i.e. the VBI of the first display system occurs sooner than that of thesecond display system), the first display system is switched to thealternate display mode (i.e. the longer scan time mode) and the seconddisplay system is maintained in the normal display mode.

Case 3: In a first and a second display system, if the normal displaymode is non-interlaced with 262 scan lines/frame and the alternatedisplay mode is interlaced with 262.5 scan lines/field (i.e. the normalmode non-interlaced scan time is shorter than the alternate modeinterlaced scan time), the first display system is synchronized with thesecond display system as follows:

a. If the first display system falls behind the second display system(i.e. the VBI of the first display system occurs later than that of thesecond display system), the first display system is maintained in thenormal display mode and the second display system is switched to thealternate display mode (i.e. the longer scan time mode).

b. If the first display system gets ahead of the second display system(i.e. the VBI of the first display system occurs sooner than that of thesecond display system), the first display system is switched to thealternate display mode (i.e. the longer scan time mode) and the seconddisplay system is maintained in the normal display mode.

Case 4: In a first and a second display system, if the normal displaymode is interlaced with 262.5 scan lines/field and the alternate displaymode is non-interlaced with 262 scan lines/frame (i.e. the normal modeinterlaced scan time is longer than the alternate mode non-interlacedscan time), the first display system is synchronized with the seconddisplay system as follows:

a. If the first display system falls behind the second display system(i.e. the VBI of the first display system occurs later than that of thesecond display system), the first display system is switched to thealternate display mode (i.e. the shorter scan time mode) and the seconddisplay system is maintained in the normal display mode.

b. If the first display system gets ahead of the second display system(i.e. the VBI of the first display system occurs sooner than that of thesecond display system), the first display system is maintained in thenormal display mode and the second display system is switched to thealternate display mode (i.e. the shorter scan time mode).

The following table (Table 1) summarizes the actions taken forsynchronizing devices in each of the four different configurations. Itwill be apparent to those of ordinary skill in the art that otherconfigurations are possible; yet, the present invention still performssynchronization as appropriate for the particular configuration.

                  TABLE 1                                                         ______________________________________                                        Case 1        Case 2    Case 3    Case 4                                      ______________________________________                                        Normal  Non-      Interlaced                                                                              Non-    Interlaced                                Display Interlaced          Interlaced                                        Mode                                                                          Alternate                                                                             Interlaced                                                                              Non-      Interlaced                                                                            Non-                                      Display           Interlaced        Interlaced                                Mode                                                                          Non-    263       263       262     262                                       Interlaced                                                                    Lines/Frame                                                                   Action if                                                                             Switch    Maintain  Maintain                                                                              Switch                                    subject subject   subject   subject subject                                   display display   display   display display                                   system falls                                                                          system to system in system in                                                                             system to                                 behind other                                                                          alternate normal    normal  alternate                                 display mode.     mode.     mode.   mode.                                     system  Maintain  Switch other                                                                            Switch other                                                                          Maintain                                          other display                                                                           display   display other display                                     system in system to system to                                                                             system in                                         normal    alternate alternate                                                                             normal                                            mode.     mode.     mode.   mode.                                     Action if                                                                             Maintain  Switch    Switch  Maintain                                  subject subject   subject   subject subject                                   display display   display   display display                                   system gets                                                                           system in system to system to                                                                             system in                                 ahead of                                                                              normal    alternate alternate                                                                             normal                                    other display                                                                         mode.     mode.     mode.   mode.                                     system  Switch other                                                                            Maintain  Maintain                                                                              Switch other                                      display   other display                                                                           other display                                                                         display                                           system to system in system in                                                                             system to                                         alternate normal    normal  alternate                                         mode.     mode.     mode.   mode.                                     ______________________________________                                    

It is normally the case that between any two similar crystal controlledsystems, one system is always slightly faster than the other andtherefore one system is always making the timing adjustments. It doeshowever, occur that one system may initially be faster, yet over time,the other system may overtake the initially faster system. Someenvironmental conditions such as temperature can cause this effect. Forexample, one system may have been powered up for a while and the othersystem was just turned on and eventually warms up. Thus, a well-designedsystem must be prepared to have either display device adjust to theother. There can be no reliance on the fact that one system may alwaysbe the faster system.

It may be desirable in an alternative display system to always remain ininterlaced or non-interlaced mode. In such cases, it is possible toadjust the synchronization of two display systems by switching a displaysystem between two field or frame sizes. Thus, for example, in a systemremaining in a non-interlaced mode, the timing adjustment apparatusswitches between a first frame size of 262 scan lines and a second framesize of 263 lines. In a system remaining in an interlaced mode, thetiming adjustment apparatus switches between a first field size of 262.5scan lines and a second field size of 263.5 or 261.5 scan lines.

It may be desirable in another alternative display system to obtain amore subtle or more finely selectable timing adjustment. In this case,the horizontal line timing can be used to adjust the synchronizationbetween two raster display systems. For example, NTSC has better chromaperformance for a fixed number of color burst clocks per horizontal linetime, so a natural time constant to vary is color burst clocks/scanline. By increasing or decreasing the horizontal line time by one colorburst clock (1/3.579545 MHz) for all scan lines in two fields or oneframe, the overall display timing can be altered to a lesser degree thanpossible by increasing or decreasing the number of scan lines scannedfor a field or frame.

PAL and SECAM standard display systems can be controlled using the sameinterlaced/non-interlaced timing adjustment described above for an NTSCsystem. The PAL and SECAM standard display systems are based on a 50 Hzfield (or non-interlaced frame) rate, with 312.5 lines per field andeither 312 or 313 lines per non-interlaced frame. PAL and SECAM standarddisplay systems can be synchronized by switching between an interlacedor non-interlaced mode in the manner described above.

Referring now to FIG. 5, a typical system architecture in which thepresent invention is used is illustrated. A data communication medium505 provides a means for transferring information between a first rasterdisplay device 510 and a second raster display device 520. In somesituations, it is necessary to synchronize device 510 with device 520 ona frame by frame basis; however, separate timing or synchronizationsignals between the two devices are not available. The present inventionprovides a means and method for synchronizing these two devices as isdescribed herein.

Referring again to FIG. 5, data communication medium 505 may be anyconventional data communication medium including a conventional localarea network (LAN), a conventional wide area network (WAN), aconventional telephone line and modem communication medium, aradio-frequency communication medium, a broadband cable televisioncommunication medium, a simple serial or parallel cable (i.e. a directconnect data communications cable), or any other well known datacommunication medium. Raster display 510 and raster display 520 are bothconventional raster scan display devices each including an interlacedand non-interlaced mode of operation as described earlier. Rasterdisplay 510 operates at a refresh rate of R₁. Raster display 520operates at a refresh rate of R₂. Even though raster display 510 andraster display 520 are comprised of identical hardware with the samenumber of scan lines and configured to operate at the same refresh rate,subtle variations in the oscillators or timing circuits of displays 510and 520 and other factors such as environmental factors contribute tocause refresh rate R₁ to be slightly different from refresh rate R₂.Thus, even if display 510 and display 520 both begin displayingidentical image frames at the same instant, eventually display 510 willbe displaying a different video frame than display 520 because of thevariations in refresh rate in R₁ and R₂. Thus, some form ofsynchronization is required to make sure that display 510 and display520 always display the same video frame.

Most computer based display controllers do not accept externalsynchronization clocks. If such clocks are provided, they are requiredto be accurate to a fraction of a pixel. For NTSC, PAL, or SECAMstandard systems, this represents an accuracy of less than 100 usec. Atypical voice-grade telephone modem (such as one based on the V.22 bisstandard) recovers its data clock with far less accuracy than onemicrosecond; thus, such a highly accurate clock is not available inthese applications. Many communication channels 505 are far tooimprecise in their timing to provide such accurate clocks. Thus, it isimpractical in most cases to a) input a synchronizing clock to thedisplay controller of devices 510 and 520, and b) if an input (such asgenlock) is available, its accuracy requirements often exceed the timingaccuracy of the available communication medium.

The present invention provides a means and a method for synchronizingthe display of images between two different raster display systems suchas display 510 and display 520 without requiring a precise clock betweenthem. The present invention uses the timing variation between interlacedmode and non-interlaced mode for correcting any timing differencesbetween the two display systems.

Referring now to FIG. 4, a flow chart illustrates the processing stepsperformed by the present invention. The synchronization processing logicof the present invention may be implemented as software or as ahardware/software implementation or purely in hardware. Various methodsfor implementing the processing logic illustrated in FIG. 4 will beapparent to those of ordinary skill in the art. The synchronizationprocessing logic of the present invention for a subject display systembegins at bubble 410 illustrated in FIG. 4. Each display system, such asdisplay systems 510 and 520, executes its own implementation ofsynchronization processing logic 410. In this example, it is assumedthat the normal display mode for both display systems is 262 lines ofhalf-resolution non-interlace (as is usually the case with Sega Genesisvideo games, for example), and the alternate display mode isfield-doubled interlace of 262.5 lines per field.

When the subject display system has completed scanning the displayscreen for a particular refresh cycle, a vertical blanking interrupt(VBI) or other refresh cycle event is generated by the display systemand received by the synchronization processing logic in processing block412. The subject display system then formulates a message indicating arefresh cycle has been completed. This message is transmitted to anotherdisplay system across data communication medium 505 in processing block414. Concurrently, the other display system on the data communicationmedium 505 is performing the same synchronization processing steps andsending messages to the subject display system on the data communicationmedium 505. After transmitting its refresh complete message on the datacommunication medium, the subject display system attempts to read arefresh complete message from the other display system in processingblock 416. If the data is available from the other display system at thetime the subject display system attempts to read the refresh completemessage or available by some expected time relative to the activation ofVBI, processing path 420 is taken to processing block 424. In this case,the subject display system has not fallen out of synchronization withthe other display system with which synchronization is desired. Atprocessing block 424, the non-interlaced mode, which is the normal modeof operation in the present example, is activated or maintained ascurrent. Synchronization processing logic then exits through bubble 430.

If the subject display system attempts to read a refresh cycle completemessage from the other display system and the message is not availableor not received within a predetermined time period, processing path 422is taken to processing block 428. In this case, the subject displaysystem has gotten slightly ahead of the other display system. Becausethe subject display system has gotten slightly ahead, the currentrefresh cycle for the other display system was completed later thanexpected as compared to the subject display system. In this case, theinterlaced mode is activated in processing block 428. The activation ofinterlaced mode lengthens the refresh cycle time required for thesubject display system for each of the next two fields by Δt asdescribed earlier in connection with FIGS. 3A and 3B (i.e., the subjectdisplay goes from 262 lines to 262.5 lines in interlaced mode). Byactivating interlaced mode in processing block 428 and therebylengthening the next refresh cycle time, the next VBI for the subjectdisplay system occurs later than it would have if the subject displaysystem had remained in a non-interlaced mode. By completing the nextrefresh cycle later, the subject display system allows the other displaysystem to catch up. In other words, if the synchronization message fromthe other display system is late, as is the case when the subjectdisplay system has gotten slightly ahead of the other system, thesynchronization message will be closer to being available when expectedfor the next frame if the synchronization message for the next frame isread half of a scan line later. As long as the synchronization messagefrom the other display system is unavailable when expected, the subjectdisplay system is maintained in interlaced mode thereby lengthening eachsubsequent refresh cycle until synchronization is achieved, and thesynchronization message is received when expected. Note that due to theinherent precision required for NTSC (for example, for colorgeneration), we are guaranteed that the two systems will not drift outof synchronization very quickly relative to each other, and as such, theone half line adjustment will only be necessary (at worst) every severalhundred frames. Note also that optional processing block 426 could beimplemented to wait for the late data if that data content is of value.In the preferred embodiment, the data content is not important forsynchronization purposes (but it is used to transfer other informationrelevant to application software running on the video game system).Rather, only the time at which the data is received is important toremain synchronized. Because drift out of synchronization is known to beslow, the wait for the late data will not be long.

Referring now to FIG. 6, a pair of time lines corresponding to a displaysystem A and a display system B is illustrated. Each of display systemsA and B display a set of frames F1 to F3. The completion of the refreshcycle for each of these frames is shown as points F1, F2, and F3 in FIG.6. On receiving the VBI at the completion of frame F1 at time t₄,display system A transmits data to display system B and sets up to readdata from display system B as indicated by line 514. Similarly, displaysystem B transmits data to display system A and sets up to read datafrom display system A as indicated by line 516. At time t₇, data isavailable from display system A as indicated by point P1. The differencebetween time t₄ and time t₇ reflects communication channel latency. Whendisplay system B is ready to read this data at the completion of frameF2, this data will still be available and held in a buffer, as indicatedby dashed line 532. Similarly, the data from display system B isavailable at point P2. Again, the difference between time t₄ and time t₇reflects communication channel latency. This data is read by displaysystem A at the completion of frame F2. This data will still beavailable for display system A as indicated by dashed line 530. Thus,according to the timing example illustrated in FIG. 6, display system Aand display system B are both in synchronization and maintained in anon-interlaced mode.

Referring now to FIG. 7, an example shows the timing correctionperformed by the present invention when the timing between displaysystem A and display system B begins to drift apart. As in the previousexample, we assume for this example that the normal display mode isnon-interlace, the alternate display mode is interlace, andnon-interlace mode produces a shorter frame time than interlace mode. Inthis example, the display of frame F1 as displayed by display system Aand display system B has drifted apart in time by the quantityillustrated by dashed line 610 illustrated in FIG. 7. Note that thetiming adjustments induced by switching to interlace mode are grosslyexaggerated in this illustration so as to highlight the operation of thepresent invention.

Display system A receives the VBI for frame F1 at time t₄. Displaysystem A then transmits data to display system B as indicated by line650 illustrated in FIG. 7. Display system B receives the VBI for frameF1 at time t₆. The difference between time t₆ and time t₄ represents theskew or the amount of time that display system A and display system Bhave drifted out of synchronization. At time t₆, display system Btransmits data to display system A as indicated by line 611. At time t₈,display system A receives a VBI for frame F2. Display system A expectsto have received data from system B by this time; however, because ofthe skew between the display systems, the data indicated by line 611 hasnot yet arrived for display system A at time t₈. Thus, display system Aswitches to interlace mode, thereby lengthening the frame time as shownby the exaggerated delayed arrival of frame F3 at time t₁₃. If displaysystem A had remained in non-interlaced mode, the VBI for frame F3 wouldhave occurred at time t₁₂. The exaggerated delayed start of frame F3 isrepresented as the difference between time t₁₃ and time t₁₂. For displaysystem B, however, the VBI for frame F2 occurs at time t₁₀. Displaysystem B expects to have received data from display system A by thistime. Indeed, the data from display system A is available for displaysystem B and held in a buffer, as indicated by line 650 and dashed line614. In this example, the data from display system A was available attime t₇. Because display system B receives the data from display systemA as expected, display system B remains in a non-interlaced mode therebymaintaining its frame length in a normal size. Thus, subsequent framesfor display system B occur at normal intervals and also at shorterintervals relative to display system A while display system A remains inan interlaced mode. As can be seen in FIG. 7, this results in data sentfrom display system B to almost arrive in time for VBI (line 617 showsdata arriving just before F3 starts), and finally arriving in time forVBI (line 618). So, in frame F4, display system A is switched to normalmode since the data from system B arrived on time, and both systemsremain in normal mode until they again drift out of synchronization anddata from one arrives too late for VBI of the other.

The example of the preferred embodiment described above in connectionwith FIG. 7 illustrates the use of the VBI as the event (refresh cycleevent) to which the expected arrival time of a synchronization messagefrom another display system is referenced. Thus, in the preferredembodiment described above, if the VBI is received at time t₀, theexpected arrival time of a synchronization message is set to time t₀.The VBI provides a convenient way of triggering the transmission of asynchronization message to another display system and for referencingthe expected arrival time of a synchronization message from anotherdisplay system.

As an alternative embodiment of the present invention, a refresh cycleevent other than the VBI itself can be used to trigger thesynchronization process. For example, a time offset can be applied tothe VBI when the expected arrival time of a synchronization message fromanother display system is computed. In this alternative embodiment, ifthe VBI is received at time t₀, the expected arrival time of asynchronization message can be set to time t₀ ±t_(offset). In thismanner, the expected arrival time of a synchronization message can beadvanced or delayed from the VBI time. This allows for more precisesynchronization. Accuracy is limited by the greater of: i) one scanline, or ii) the timing precision of the communication medium.

In another alternative embodiment, a refresh cycle event other than theVBI or an offset thereof can be used. For example, a refresh cycle eventbased on the number of scan lines scanned for a particular frame canserve as a trigger event for computing an expected time for receipt of asynchronization message. Using this alternative embodiment, a refreshcycle event can be defined as occurring when the current frame has beenpartly scanned. For example, a refresh cycle event can be generated whenscanning of the current frame is 50% or 25% complete. This alternativeembodiment is useful if the VBI is unuseable or inconvenient for use asa refresh cycle event. Also, many video game systems (such as the SegaGenesis system) can generate interrupts on particular scan lines. Theseinterrupts can also be used as a refresh cycle event.

Note that in all cases, the expected arrival time can be advanced ordelayed by more than one frame time. Thus, the latency in the receipt ofthe synchronization message can be several frame times without loss ofsynchronization using the present invention.

It may be desirable to accommodate imprecise timing references fordetermining whether displays are "ahead or behind" relative to eachother. This can be easily accomplished by specifying a threshold forswitching to alternate mode which is larger than the worst case jitterof the timing reference. Thus, even with an imprecise timing reference,both displays will usually remain in normal mode.

The oscillators in crystal controlled display systems provide areasonably accurate timing source. Two or more systems operating withdifferent oscillators will certainly drift out of synchronizationeventually; however, the rate of drift will typically be rathersmall/slow. For this reason, intermittent short interruptions in theoperation of the communication medium between display systems beingsynchronized using the present invention can be tolerated. In addition,loss of the refresh cycle event timing reference for short duration canalso be tolerated by the present invention. In both of these situations,the synchronization process described above is continued once theoperation of the communication medium or the timing reference isre-established. Interruptions of up to two seconds can usually betolerated in most situations.

The present invention can also be used in a communications environmentwith asymmetric latency. The present invention is affected only by thelatency in each direction of information transfer independently. So longas the latency each way is consistent, the measurement of the dataarrival time in each direction will provide a sufficient time referencefor each respective display system. The refresh cycle events of the twodisplay systems may settle into relative positions where they are skewedfrom one another; but, they will remain locked together without one evergetting a frame ahead of the other.

The present invention can also tolerate periodically varying latency. Ifa communications channel is known to have gradually varying latency(e.g. if a data path is periodically re-routed due to traffic) and thereis a means to detect that the latency has changed, the present inventioncan switch its timing measurement to a different arrival time (as anexample) to accommodate the new latency. Even if there is a brief lossof timing reference while the latency is changing, the crystalcontrolled timing on the display generators will keep the two displaysystems in synchronization with each other.

The present invention can also be used in a single raster display as anapparatus for adjusting the timing of the raster display. It issometimes necessary to synchronize the refresh timing of a rasterdisplay with a secondary timing event, rather than with another rasterdisplay. Such secondary timing events may originate from within theraster display itself or they may be external timing sources. Suchsecondary timing sources can include a timing source derived from a 60Hz power line on a common power grid, a periodic satellite pulse, abroadcast timing signal, an alternate timing source within a computersystem, or another secondary timing source. In the same manner describedabove for the synchronization of a first raster display with a secondraster display, a single raster display can also be synchronized withthe secondary timing source. Specifically, the present inventionincludes a means for detecting a refresh cycle event in the rasterdisplay, just like in the multiple raster display synchronizationimplementation. In addition, the present invention includes a means fordetecting the secondary timing event comprising a signal path or aninterface for receiving a secondary timing signal. The time ofoccurrence of the refresh cycle event is compared with the time ofoccurrence of the secondary timing event. If the refresh cycle of theraster display drifts out of synchronization with the secondary timingevent, the present invention activates an alternate display mode in theraster display to reestablish synchronization of the raster displayrefresh cycle with the secondary timing source. In a manner similar tothe method and apparatus described above for applying a timing offset tothe expected message arrival time calculation, a timing offset can alsobe applied to the secondary timing event prior to being compared withthe refresh cycle event. This provides better control over thesynchronization process.

In an alternative embodiment of the apparatus and method for adjustingthe refresh timing in a single raster display, the secondary timingevent can also be received by the raster display via a datacommunications medium, such as a network or a direct connect cable.Secondary timing information is coded into a message received andinterpreted by the present invention in the raster display. The presentinvention then compares the secondary timing information thus receivedwith the refresh cycle timing of the raster display and activates analternative display mode to synchronize the two timing sources.

In yet another alternative embodiment of the apparatus and method foradjusting the refresh timing in a single raster display, the refreshtiming of the raster display is averaged over multiple cycles. After theaverage refresh cycle timing is determined, the average can be adjustedby periodically switching the raster display to an alternate displaymode. One example of an application for such an averaging technique isfor synchronizing a raster display with a sound generator in amultimedia capable computer system. In such multimedia systems, it isnecessary to synchronize the timing of the raster display with thetiming of the sound generator clock so that the video and audiopresentations will be aligned. If the timing differential between theaudio and video presentations is known, the raster timing, on average,can be adjusted using the present invention by periodically switchingthe raster display to an alternate display mode. In this manner, theaverage refresh cycle time for the raster device can be shortened orlengthened to conform with the audio timing.

In the preferred embodiment of the present invention for synchronizingtwo or more raster display systems, the content of the synchronizationdata transferred between systems is not important for synchronizationpurposes. Rather, only the time at which the data is received isimportant for synchronization. However, in an alternative embodiment ofthe present invention, the content of the synchronization datatransferred between systems can be used to assist the synchronizationprocess. In this embodiment, the synchronization data itself containsinformation defining the timing relationship between the refresh cycleevent and a secondary timing reference. For example, the timingrelationship information can include data defining the skew between theVBI of the raster display system and a secondary timing source. As setforth above, the secondary timing sources can include a timing sourcederived from a 60 Hz power line on a common power grid, a periodicsatellite pulse, a broadcast timing signal, an alternate timing sourcewithin a computer system, or another secondary timing source. The timingrelationship information is encoded into a synchronization message by afirst raster display and sent to a second raster display across a datacommunications medium. The second raster display decodes thesynchronization message and adjusts its own refresh cycle timingaccordingly, if necessary, by entering an alternate display mode. Notethat in this embodiment, the arrival time of the synchronization messageis irrelevant to the synchronization process.

In yet another embodiment of the present invention, the synchronizationprocess described herein can be used to synchronize multiple rasterdisplays arranged in a daisy chain configuration. In this embodiment, itis desired to have all raster display systems synchronized together.However, it is undesirable to require all raster displays to communicatewith all other raster displays for the purpose of synchronization. Thepresent invention allows each raster display system to communicate withone and only one other raster display for the purpose ofsynchronization. If each raster display in the daisy chain has thecapability to slow its refresh cycle by entering an alternate displaymode, a raster display that begins to get ahead of the one other rasterdisplay with which it is communicating switches to the alternate displaymode to regain synchronization with the slower raster display. The otherraster displays in the daisy chain will correspondingly slow down byusing their alternate display modes. Eventually, each raster display inthe daisy chain will be synchronized with the slowest raster display ofthe group. If each raster display in the daisy chain has the capabilityto speed up its refresh cycle by entering an alternate display mode, araster display that begins to get behind one of the other rasterdisplays with which it is communicating switches to the alternatedisplay mode to regain synchronization with the faster raster display.The other raster displays in the daisy chain will correspondingly speedup by using their alternate display modes. Eventually, each rasterdisplay in the daisy chain will be synchronized with the fastest rasterdisplay of the group.

Thus, an apparatus and method for synchronizing closed free-runningsystems is disclosed. These specific arrangements and methods describedherein are merely illustrative of the principles of this invention.Numerous modifications in form and detail may be made by those ofordinary skill in the art without departing from the scope of thepresent invention. Although this invention has been shown in relation toa particular preferred embodiment, it should not be considered solimited. Rather, the present invention is limited only by the scope ofthe appended claims.

We claim:
 1. An apparatus for adjusting the timing of the display offrames in a raster display having an interlaced display mode and anon-interlaced display mode, said apparatus comprising:means fordetecting a refresh cycle event in the raster display operating in oneof said interlaced display mode and said non-interlaced display mode;means for receiving information indicative of a secondary timing event,said information being received from an external timing reference; meansfor comparing a time of occurrence of said refresh cycle event with saidinformation indicative of said secondary timing event; and means foractivating an alternate one of said interlaced display mode and saidnon-interlaced display mode in said raster display to synchronize thedisplay of frames in said raster display in reference to said secondarytiming event.
 2. The apparatus as claimed in claim 1 wherein said firstraster display is an NTSC type display.
 3. The apparatus as claimed inclaim 1 wherein said first raster display is a PAL type display.
 4. Theapparatus as claimed in claim 1 wherein said first raster display is aSECAM type display.
 5. The apparatus as claimed in claim 1 wherein saidapparatus is coupled to a video game system.
 6. The apparatus as claimedin claim 1 wherein said apparatus is coupled to a first video gamesystem directly and coupled to a second video game system via modem. 7.An apparatus for adjusting the timing of the display of frames in araster display operating normally in a non-interlaced display mode, saidapparatus comprising:means for detecting a refresh cycle event in araster display; means for receiving information indicative of asecondary timing event, said information being received from an externaltiming reference; means for comparing a time of occurrence of saidrefresh cycle event with said information indicative of said secondarytiming event; and means for periodically activating an interlaceddisplay mode in said raster display to synchronize the display of framesin said raster display in reference to said secondary timing event. 8.The apparatus as claimed in claim 7 wherein said first raster display isan NTSC type display.
 9. The apparatus as claimed in claim 7 whereinsaid first raster display is a PAL type display.
 10. The apparatus asclaimed in claim 7 wherein said first raster display is a SECAM typedisplay.
 11. The apparatus as claimed in claim 7 wherein said apparatusis coupled to a video game system.
 12. The apparatus as claimed in claim7 wherein said apparatus is coupled to a first video game systemdirectly and coupled to a second video game system via modem.
 13. Anapparatus for adjusting the timing of the display of frames in a rasterdisplay operating normally in an interlaced display mode, said apparatuscomprising:means for detecting a refresh cycle event in a rasterdisplay; means for receiving information indicative of a secondarytiming event, said information being received from an external timingreference; means for comparing a time of occurrence of said refreshcycle event with said information indicative of said secondary timingevent; and means for periodically activiating a non-interlaced displaymode in said raster display in reference to said secondary timing event.14. The apparatus as claimed in claim 13 wherein said first rasterdisplay is an NTSC type display.
 15. The apparatus as claimed in claim13 wherein said first raster display is a PAL type display.
 16. Theapparatus as claimed in claim 13 wherein said first raster display is aSECAM type display.
 17. The apparatus as claimed in claim 13 whereinsaid apparatus is coupled to a video game system.
 18. The apparatus asclaimed in claim 13 wherein said apparatus is coupled to a first videogame system directly and coupled to a second video game system viamodem.
 19. An apparatus for synchronizing raster displays, the rasterdisplays being coupled by a data-communications means, said apparatuscomprising:a first raster display; a means for sending signals from thefirst raster display; a second raster display, the second raster displaycapable of operation in a non-interlaced display mode and an interlaceddisplay mode, the second raster display having a means for receivingsignals, a means for changing the operational mode of the second rasterdisplay from non-interlaced display mode to interlaced display mode inresponse to a first signal sent from the first raster display; and ameans for changing the operational mode of the second raster displayfrom interlaced display mode to non-interlaced display mode in responseto a second signal sent from the first raster display.
 20. The apparatusas set forth in claim 19 wherein the first raster display is capable ofoperation in a non-interlaced display mode.
 21. The apparatus as setforth in claim 19 wherein the first raster display is capable ofoperation in an interlaced display mode.
 22. The apparatus as set forthin claim 19 wherein the first raster display is capable of operation ina non-interlaced mode and an interlaced display mode.
 23. The apparatusas set forth in claims 19, 20, 21 or 22 further comprising a means forsending a signal from the second raster display, and the first rasterdisplay having a means for receiving signals.